Lots of good answers in this thread so far.
I learned more about the art of software development, programming, testing and documenting in the first month of contributing/working on an open source project than I'd learned in about 5 years working in software companies.
Its hard to quite explain really why this is other than if you find a reasonably popular well run open source project, you tend to find many like minded peers who love to both share useful ideas and information as well as learn new things and push the boundaries of the art. You also get to read lots of existing code and documentation and see it being changed in real time to help learn new ideas and approaches.
Programming is such a broad topic from design, testing, technologies, frameworks, APIs, building tools, documentation, IDEs, patterns and being lean & agile to name but a few off the top of my head - its kinda hard to pick up this vast landscape from a few books or courses or to figure this stuff out all yourself; its better to just watch some highly experienced folks demonstrate it all in action on an open source project.